The 2003 Nobel prizes in physics and physiology or medicine both have connections with the field of particle physics. Alexei Abrikosov, Vitaly Ginzburg and Anthony Leggett have received the physics prize for “pioneering contributions to the theory of superconductors and superfluids”, while Paul Lauterbur and Peter Mansfield were rewarded for discoveries in magnetic resonance imaging (MRI), which is in turn a major use for superconducting magnets.
The most important superconducting materials technically have proved to be type-II superconductors, which allow superconductivity and magnetism to exist at the same time. Type-I superconductors expel a magnetic field in what is known as the Meissner effect, and lose their superconductivity at high magnetic fields. However, type-II superconductors – generally alloys of various metals – exhibit only a weak Meissner effect, or none at all, and retain their superconductivity at high magnetic fields. Superconducting magnets are now routinely used in particle accelerators, and the magnets used for CERN’s Large Hadron Collider (LHC) are based on coils of niobium-titanium alloy, a type-II superconductor.
Abrikosov, who is now at Argonne National Laboratory, was working at the Kapitsa Institute for Physical Problems in his native Moscow when he succeeded in formulating a new theory to explain the behaviour of type-II superconductors, which cannot be explained by the earlier BCS theory (Nobel prize 1952). In Abrikosov’s theory, the external magnetic field penetrates the type-II material through channels within vortices in the “electron fluid” in the material. This theory was based on work in the 1950s by Ginzburg at the P N Lebedev Physical Institute in Moscow.
Superfluidity is another low-temperature phenomenon that will become large scale with the LHC. Liquid helium, which is used to cool superconducting magnets, becomes superfluid at temperatures below its boiling point and takes on heat transfer properties that allow efficient heat removal over the large distances involved in the LHC. The helium used to cool the LHC magnets is the common isotope, helium-4, which becomes superfluid around 2 K. However, helium also exists as a rarer isotope, helium-3, which is superfluid only at much lower temperatures of mK. While helium-4 is a boson, helium-3 is a fermion, so the two isotopes have quite different quantum properties. The contribution of Leggett, now at the University of Illinois, Urbana, was to develop the decisive theory, while at Sussex in the UK in 1970s, to explain how helium-3 atoms interact and become ordered in the superfluid state.
There is a link between this year’s physics prize and the prize in medicine, as one of the major uses for superconducting magnets is in MRI. The nuclear resonance phenomenon used in MRI was first demonstrated, for protons, in 1946 by Felix Bloch and Edward Mills Purcell, who received the Nobel prize in 1952. In a further connection with particle physics, Bloch went on to be the first director-general of CERN until 1955. It was 20 years, however, before Lauterbur, from Urbana, Illinois, discovered in 1973 how to create 2D pictures by introducing gradients in the magnetic field. Peter Mansfield of Nottingham in the UK developed this idea further by showing how the resonance signals could be mathematically analysed to make a useful imaging technique. He also demonstrated how extremely fast imaging could be achieved. Today, MRI is used to examine almost all organs of the body, and is especially valuable in imaging the brain and spinal cord.
